1. Field of the Invention
The present invention relates to a sensor device, in which a sensor element having a displace part is mounted on a board through a resin member.
2. Description of Related Art
JP-A-8-35983 discloses a sensor device including a board and a sensor element. The sensor element includes a displace part to be displaced in a predetermined detection direction, and detects a displace amount of the displace part in the detection direction. The sensor element is mounted on the board through a resin member made of resin, and the sensor element is connected to the board through the resin member.
Here, the board is made of ceramic or resin, or the board is constructed by a semiconductor substrate. The sensor element represents an acceleration sensor element or an angular speed sensor element, for example. The sensor element includes a movable part as the displace part, and the movable part is displaced in the detection direction due to acceleration or angular speed.
Further, adhesive made of epoxy resin is used as the resin member. The sensor element is mounted on the board through the resin member, and the resin member is heated to be hardened.
FIGS. 6A, 6B and 6C show a conventional sensor device. FIG. 6A is a plan view showing the conventional sensor device. FIG. 6B is a schematic cross-sectional view showing the conventional sensor device taken along line VIB-VIB in FIG. 6A. FIG. 6C is a schematic cross-sectional view showing the conventional sensor device taken along line VIC-VIC in FIG. 6A.
The conventional sensor device includes a sensor element 30 and a board 20. The sensor element 30 includes a displace part 31, and is fixed on the board 20 through a resin member 40. The displace part 31 is displaced in a detection direction Y. The resin member 40 is arranged under the whole sensor element 30.
Experiments and simulations are performed by using the conventional sensor device. When a temperature variation is generated, the sensor element 30 is warped as shown in FIGS. 6B and 6C, although the sensor element 30 is originally flat. Specifically, because end portions 30a, 30b of the sensor element 30 are fixed to the resin member 40, middle part of the sensor element 30 is warped to protrude outward. A reason for the warp of the sensor element 30 will be described below.
When the resin member 40 is heated to connect the board 20 and the sensor element 30, each of the board 20, the sensor element 30 and the resin member 40 expands. In contrast, when the resin member 40 is cooled toward a room temperature, each of the board 20, the sensor element 30 and the resin member 40 contracts. The temperature variation is generated when the conventional sensor device is manufactured. In addition to the manufacture time, the temperature variation is generated when the conventional sensor device is used. Due to an environmental temperature variation, each of the board 20, the sensor element 30 and the resin member 40 expands or contracts.
Here, the expansion and the contraction of the sensor element 30 are smaller than those of the resin member 40 and the board 20, because the sensor element 30 is made of semiconductor. Therefore, when the temperature variation is generated, the sensor element 30 is warped due to a thermal deformation of the resin member 40 and a thermal expansion difference between the sensor element 30 and the board 20.
In this case, as shown of arrows in FIG. 6A, the sensor element 30 is warped in almost all directions including the detection direction Y. Because almost all peripheries of the end portions 30a, 30b of the sensor element 30 are fixed to the resin member 40, stress of the resin member 40 is applied to an approximately center part of the sensor element 30 from the almost all peripheries of the end portions 30a, 30b of the sensor element 30.
FIG. 6B shows the warp of the sensor element 30 in a direction X approximately perpendicular to the detection direction Y. FIG. 6C shows the warp of the sensor element 30 in the detection direction Y. Here, when the sensor element 30 is cut parallel to the direction X, Y in a thickness direction, the warp of the sensor element 30 in the direction X, Y represents that a center part 30c of the cross-section of the sensor element 30 departs from an imaginary line L. The imaginary line L is defined to connect ends of the cross-section in the direction X, Y, and represents a flat state of the sensor element 30 not having the warp.
If the sensor element 30 is warped in the detection direction Y, deviation or error may be generated in the displace amount of the displace part 31, and the displace amount of the displace part 31 may not accurately be detected. For example, a capacitive physical quantity sensor includes a movable part to be displaced in the detection direction Y when a physical quantity, e.g., acceleration is applied to the sensor. The physical quantity can be detected based on a variation in a distance between the movable part and a fixed part opposing to the movable part in the detection direction Y.
Here, when the sensor element 30 is warped in the detection direction Y, the variation in the distance between the movable part and the fixed part, that is, the displace amount, departs from a designed relationship between the physical quantity and the displace amount. Thus, error may be generated in the detection accuracy. Especially, if the sensor element 30 is warped due to the temperature variation when the sensor device is used, the warp of the sensor element 30 may affect the detection accuracy.